Detectives use it almost in every case. It's collecting details about individual or firm, criminal records of person, negative moments in company activity. Also you may need to find real address, phone of person, check some "new" car or house which you want to buy (maybe they have unknown owners and you'll have problems then...) and more and more.
Price - depends on volume and subject. Standart dossier about individual - $150, about company - $250
Periods - official information 1-3 days. But if we must to find out actually details, which won't be in official documents (real home, work address, parking place etc.) - about 3-7 days.